INTENSITY OF SUN'S HEAT AND LIGHT. 



37 



years prior to A. D. 1800. The eccentricity of the orbit, e', was then .0187, accord- 

 ing to Leverrier ; and for the obliquity of the ecliptic, the most correct formula is 

 probably that of Struve and Peters, quoted in the American Nautical Almanac. It 

 is true, their formula may not strictly apply for so distant a period ; but, since the 

 value 24° 43' falls within the maximum assigned by Laplace, it must be a com- 

 patible value, though its epoch may be somewhat nearer or more remote than 10,000 

 years. Therefore, substituting this value of co, 24° 43' in equations (30), (31) and 



multiplying by f -, in order to substitute the proper eccentricity, and com- 

 paring the computed results with the table for 1850, given in Section V, as a 

 standard, we find the annual intensity on the equator, at the former period, to have 

 been 1.65 thermal days less than in 1850 ; the differences for every ten degrees of 

 latitude are as follows: — 



Change of the Sun's Annual Intensity 8,200 Years B. 0., from its Value in A.D. 1850, taken as 



the Standard. (Plate III.) 



Latitude 



! 

 Difference in thermal days. 



Latitude. 



Difference in thermal days. 



Latitude. 



Difference in thermal days. 



0° 

 10° 

 20° 



—1.65 

 —1.58 

 —1.32 



30° 



40° 

 50° 



—.96 

 —.22 

 + .68 



60° 



70° 

 80° 

 90° 



+ 2.11 



+ 5.52 

 + 7.18 

 + 7.64 



These results are exhibited graphically also on Plate III ; from which it appears 

 that the annual intensity within the Torrid Zone ten thousand years ago, averaged 

 one thermal day and a half less than now ; while from 35° of latitude to 50°, com- 

 prehending the whole area of the United States, it was virtually the same as at the 

 present day. But above 50° of latitude, the annual intensity was then greater in 

 an increasing rate towards the Pole, at which point it was between seven and eight 

 thermal days greater than at the present time ; in other words, the Poles both North 

 and South, 10,000 years ago received twenty rays of solar heat in a year, where 

 they now receive but nineteen. Owing to change in the obliquity of the ecliptic, 

 the Sun may be compared to a swinging lamp; at the former period, it apparently 

 moved farther to the north and to the south, passing more rapidly over the inter- 

 mediate space. 



The maximum variation of the obliquity of the ecliptic according to Laplace, 

 without assigning its epoch, is 1° 22' 34", above or below the obliquity 23° 28' in the 

 year 1801. 1 Now the difference recognized in our calculation almost reaches this 

 limit, being 1° 15'. As the secular perturbations are now understood, therefore, it 

 follows that, since the Earth and Sun were placed in their present relation to each 

 other, the annual intensity upon the Temperate zones has never varied (Plate III) ; 

 between the Tropics, it has never departed from its present annual amount by more 

 than about ^ th part, and is now very slightly increasing. The most perceptible 



1 M^canique Celeste, Vol. II, p. 856, note, Bowditch's translation. 



